Kagan has shown that an infant's "temperament" is quite stable over time, in that certain behaviors in infancy are predictive of certain other behavior patterns in adolescence[3] He did extensive work on temperament and gave insight on emotion. Kagan was listed as the 22nd most eminent psychologist of the 20th century, just above Carl Jung.[4]

After he graduated from Yale University, he accepted his first faculty position at Ohio State University.[5] Six months later, in 1955 he was recruited to be a part of the research team at the U.S. Army Hospital during the Korean War.[5] Once he had finished his time at the U.S. Army Hospital, the director of the Fels Research Institute contacted Kagan to ask him to direct a project that was funded by the National Institutes of Health, which he accepted.[5] After that project was completed, he accepted the offer he received from Harvard University to be involved in creating the first Human Development program.[5] Once he moved to Harvard, he remained there until retirement, with the exception of a leave to go and study children in San Marcos.[5] He did this for a year, from 1971 to 1972, and then returned to Harvard as a professor.[5]

While at Fels, Kagan did extensive research on personality traits beginning with infancy and continued through adulthood.[8] He looked at whether or not early experiences affected the participants' future personalities, talents and characters.[5] Kagan read up on all of the longitudinal information that was prepared, specifically, the responses to intelligence tests that were administered to them.[5] When Kagan was reviewing the material collected in childhood and adulthood, he found that the first three years in childhood showed little relation to the data collected in adulthood.[5] The results of the Fels study was discussed in Kagan's book, Birth to Maturity, in 1962.[5] Kagan's next research was in San Marcos, Guatemala.[5] During this time, Kagan discovered that biological factors play a huge role in development and an even larger part in child development.[9] Specifically, he found that these children had slower Psychological development when in their homes due to their restricted experiences.[5] Once the children were walking and could leave the home, Kagan found that the psychological delay in development was only temporary, and that cognitive growth is malleable.[5] In 2010, Kagan was involved in a similar study that focused on specific parts of the brain involved in behavioral inhibition in infants. Schwartz et al. (2010) performed a longitudinal study involving 18-year-olds and used neuroimaging to detect whether or not the ventromedial or orbitofrontal cerebral cortex are associated with the high/low reactivity of their 4-month-old selves. After undergoing a battery of tests, the infants were later categorized into two groups: low-reactive and high-reactive temperament. Results showed that the adults who had low-reactive infant temperaments had greater thickness in the left orbitofrontal cortex than the high-reactive group. The adults categorized as high-reactive infants displayed greater thickness in the right ventromedial prefrontal cortex. While at Harvard University, Kagan studied infants up to two years and published his work in his book, The Second Year.[5] Kagan's research found that there were major changes in Psychological functioning between 19 and 24 months, and that one year old children were sensitive to events that deviated from their normal experiences.[10] Kagan also looked at the effects of infant daycare in response to Congress' proposal to fund federal day care centers for working mothers.[5] Richard Kearsley, Philip Zelazo and Kagan created their own daycare in Boston's Chinatown, and compared these infants to infants who stayed at home with their mothers.[11] This research revealed very little difference between the infants in day care and those whose mothers stayed at home with them with respect to cognitive functioning, language, attachment, separation protest, and play tempo[11]

Kagan proposed that emotion is a psychological phenomenon controlled by brain states and that specific emotions are products of context, the person’s history, and biological make-up.[12] Kagan also explained emotion as occurring in four dinstinct phases, including the brain state (created by an incentive), the detection of changes in bodily movement, the appraisal of a change in bodily feeling, and the observable changes in facial expression and muscle tension.[12] These emotions vary in magnitude and usually differ across ages and when expressed in different contexts.[12] Kagan questioned relying on individual's verbal statements of their feelings.[12] He provided several reasons for this; he argued that the English language does not have enough words to describe all emotional states, the words to explain emotional states do not convey the differences in quality or severity, and translating emotion words from one language to another produces variations and inaccuracies.[12][12] In addition, Kagan argued that research in emotion studies should be free of ambiguous and coded terms, and this emphasis on specificity remains a recurring theme in his current research on emotion.[12]

Temperament is perhaps what Kagan is best known for. He began his work on temperament after his research in Guatemala. Kagan primarily focused on children’s fear and apprehension.[14] He defined two types of temperament; inhibited and uninhibited.[15] Inhibited refers to a shy, timid, and fearful profile of a child, whereas uninhibited refers to the appearance of bold, sociable and outgoing behaviours.[15] In 2008, Kagan and several other researchers conducted a study to examine if behavioral inhibition in adulthood can be predicted by certain behavioral characteristics in infants.[16] The research hypothesized that the frequency of infant reactivity based on motor and crying dimensions is predictive of behavioral inhibition. As a result of his ground breaking work on temperament, we know that these characteristics have the ability to influence later behavior depending on how they interact with the environment.[14] Kagan also believed that there is no guarantee of an indefinitely stable profile considering environmental factors are always changing and that both genes and environmental factors influence a child's temperament[15]